There are three widely used methods for plating metal onto a substrate, and in some recent developments of plating circuitry on a non-conductive flexible plastic substrate all three may in fact be employed. These methods are (1) electroless plating, (2) immersion plating, and (3) electroplating. The choice of which method to employ for a given plating step is often controlled as much by the choice of the substrate or disadvantages of the respective methods as vis-a-vis the other methods. Although both methods (1) and (2) may be broadly characterized as electroless plating, this latter term as used hereinafter will be restricted to its narrower definition (involving reduction of the plating metal from the plating bath, as discussed below).
Some of the electroplating disadvantages are: a conductive path is required for the desired circuit pattern; due to uneven current densities, the "throwing power" of electrolytic baths can never reach the desired uniformity of deposit; e.g. with irregularly shaped substrates. The alkaline stannate electrolytic baths have greater throwing power than the acid stannous baths and require less critical control, but because these are restricted to the four valent stannate form these have a lower cathode efficiency.
Electrolytic deposits of tin are corrosion resistant and non-toxic, possess good solderability with good softness and ductility. However, to provide corrosion protection of a substantial nature the tin deposits should be thick enough to be virtually non-porous. Recommended thicknesses by the Tin Research Institute for plating of tin on copper is 0.5 mils both for soldering and for resisting atmospheric corrosion.
Tin, or even tin-lead alloys, are electrolytically deposited in thicknesses typically from 0.2 to 2.0 mils when used in printed and other circuitry to provide a solderable finish, a contact material, or an etchant resist. Tin can be readily deposited from acid solutions at room temperature. Where a lower melting point material is required, tin-lead alloys, such as the typical 60%-40% solder can also be deposited, but require much closer control of the solution composition and operating conditions, making it more costly (see Printed and Integrated Circuitry, Schlabach and Rider, McGraw-Hill Book Company, Inc., New York, 1963, at page 146).
Electroless plating involves the use of a plating bath without the imposition of any electric current where the substrate is plated by reduction of the plating metal from a solution of a salt of a plating metal. The plating solution contains controlled reducing agents which are generally either catalyzed by the surface of the substrate, or by some catalytic metal emplaced onto the surface both to initiate the reduction and to give good adherence. Since the plated-on-surface is autocatalytic, an electroless process can be used to build up good thicknesses. Furthermore since an electroless process is not dependent upon current densities, the resulting coating is of excellent uniformity.
However, reducing agents in electroless baths must be controlled in order to avoid spontaneous reduction of the metal in the bath e.g. to a fine powder. The reduction is not localized at the surface of the substrate, hence considerable losses may occur. Moreover, with copper-based substrates the electroless tin baths are affected adversely by contaminants such as cyanides, lead, zinc, manganese, and cadmium (see Metals Handbook, Vol. 2., p. 642; copyright 1964; American Society for Metals).
Immersion plating, like electroless plating does not employ an electric current. Immersion plating, sometines called galvanic plating, is an electrochemical displacement reaction which depends on the position that the substrate metal occupies in the electromotive series with respect to the metal to be deposited from solution. Plating occurs when the metal from a dissolved metal salt is displaced by a more active (less noble) metal that is immersed in the solution. This requirement indicates one limitation to the process, since in the standard electromotive series test conditions, copper is more noble than tin and under those conditions could not be placed by an immersion process to give a tin plate coating. However under acidic conditions, the relative electrode potentials reverse making the utilization of this process possible.
Major limitations of immersion plating in the past have been in the slow plating speeds and limited thicknesses. Immersion plating is generally self limiting, because as the plating coating builds up it tends to mask the underlying base metal thereby preventing further replacement; additionally as the displaced base metal is dissolved into the solution, it becomes an increasing contaminent in the bath progressively slowing the rate of displacement. Normal deposit thickness from immersion tin processes of the prior art is 50 to 100 microinches (i.e., 0.05-0.1 mils), mainly because of the foregoing problems in building the deposit to greater thicknesses.
An advantage of the immersion processes over many of the other aforementioned processes is the absence of hydrogen generation (or other gases) on the plating surface, thereby avoiding pitting or similar plating discontinuties. Also the immersion plating process is not subject to the surface roughness found in electroplating due to "drag-over" from precleaners, anode corrosion, and the like.
In comparison to electroless baths, there is no problem with the bath decomposing. With immersion plating, the circuit need not be electrically continuous nor need it have electrical contacts attached. There is no need for maintaining a precise current and the deposit is uniform in thickness. However in spite of these latter advantages, the prior art generally dismissed such immersion plating process for use with printed circuitry because it was noted that deposition of the more noble metal continued only in the presence of exposed base metal so that such deposits were "limited in thickness, porous, and often poorly adherent" and therefore of "limited interest" (see Printed and Integrated Circuitry, supra, at page 138).
Tin and some solders are subject under certain conditions to growth from their surfaces of metallic filaments known as "whiskers." These dendritic formations can in time and under proper conditions project from the surface to a length (as much as one-fourth of an inch) sufficient to short out adjacent circuitry when used for fine resolution electronic application printed circuits. The growth rate is encouraged slightly by elevated temperature and humidity, but is greatly promoted by high stresses (causing growth in a matter of hours). Such high stresses occur in thin tin or solder coatings (less than about 100 microinches) and will be higher in electrodeposited coatings because of the stresses which are impressed by the flow of current, not present in the electroless and immersion coating methods. As the prior art immersion coating method for depositing tin on copper are self limiting generally at a thickness of below 100 microinches, to a maximum limited to approximately 200 microinches; tin plating from the prior art immersion methods can be particularly susceptible to whisker growth not just in the original formation but in subsequent use.
Applicant has been unable to find any immersion process for plating a tin-lead alloy in the literature, and is aware of only one system, recently available on the market (a fluoroborate and immersion process) which codeposits any lead, and that only less than 1% (at 20-30 microinches of the tin maximum in 20 minutes at an operating range of 125.degree.-150.degree. F.). The lead present in the latter system appears to have no effect on the rate of deposit and is intended solely to give a cosmetic effect.
The self limiting feature of prior art immersion tin plating procedures also made it more difficult to solder the resulting tin plating because of its limited thickness.
From the foregoing it can be appreciated that due to various disadvantages inherent in electroplating and electroless plating, immersion plating would for a great many applications be a more desirable method if a greater plating rate could be achieved and if a thicker final plating coating were possible, all with reduced whisker growth. Therefore objects of the present invention include methods and baths for immersion plating which overcome or minimize by a great order of magnitude each of these aforementioned limitations. More specifically, an object of the present invention is a novel method giving a faster rate of immersion plating so as to be more competitive with the other types of plating methods. A further object is with an immersion plating method giving essentially tin or tin-lead alloys of commercially useful thickness of the order of magnitude including 0.5 mils.